When replacing file storage subsystems capable of storing electronic files used for businesses and sharing the same using a network, it is necessary to migrate the data (files, directory structures and the like) stored in existing file storage subsystems to a new file storage subsystem.
Generally, in such cases, a large amount of data is accumulated in the existing file storage subsystem being the migration source, and it takes a long time to migrate all the data. Therefore, in the prior art, a technique called online migration is known, where file operation and the like can be performed successively without stopping operation during the data migration processing, as taught in Patent Literatures 1 and 2.